1. Field of the Invention
Example embodiments of the present invention relate in general to a tracking signal generating apparatus for optical signal reproduction, and to a digital phase controller of the apparatus.
2. Description of the Related Art
FIG. 1 is a schematic view of an optical reproducing apparatus 10. Referring to FIG. 1, the optical reproducing apparatus 10 includes a spindle motor 600 for rotating an optical disc 100. The optical disc 100 is an optical information storage medium. Apparatus 10 includes an optical pickup 200 which is movably installed in the radius direction of the optical disc 100 and reproduces information recorded on the optical disc 100. The apparatus 10 also includes a driver 400 for driving the spindle motor 600 and the optical pickup 200, a signal processor 300 for detecting various signals including a tracking signal using a detection signal received from an optical detector (not shown) of the optical pickup 200, and a controller 500 for controlling a tracking servo operation of the optical pickup 200 using a signal detected by the signal processor 300.
As data is to be stored in the optical disc 100, detecting a tracking signal is essential in performing a stable servo operation. Generally, when data is reproduced from an optical storage medium such as a digital versatile disk (DVD), a blue ray disk (BD), or a high-definition DVD (HD-DVD), a pit depth of the disc is set to a ¼ wavelength due to certain limitations in conventional tracking methods based on a given signal-to-noise ratio and certain compatibility with CDs, etc. Accordingly, since a push pull method (a conventional method primarily used for detecting a tracking signal in CDs) cannot be used, a differential phase detection (DPD) method is used for detecting a tracking signal.
FIG. 2 is a block diagram of the signal processor 300 illustrated in FIG. 1. Referring to FIG. 2, the signal processor 300 includes an optical detector 301, comparators 303a, 303b, 303c and 303d, phase detectors 304a and 304b, and an integrator 305.
In a tracking signal generating method based on a DPD process, signals A, B, C and D received by the optical pickup 200 are sent to the optical detector 301, amplified by an amplifier (not shown) and then transferred to comparators 303a through 303d. Comparators 303a through 303d compare the signals A, B, C and D with a reference value and generate square wave signals INA, NB, INC and IND.
The phase detectors 304a and 304b detect phase differences between the square wave signals INA, INB, INC and IND, that is, a phase difference between the signal INA and the signal INB and a phase difference between the signal INC and the signal IND. The integrator 305 integrates the detected phase differences and generates a tracking signal (hereinafter, referred to as a DPD tracking signal) based on the DPD process.
If switches 302 are open, a four-channel type DPD tracking signal is generated. If the switches 302 are closed, a two-channel type DPD tracking signal is generated. In order to generate the DPD tracking signal, the phase detectors 304a and 304b use a SR latch method and a D flip flop method. Therefore, the phase detectors 304a and 304b normally operate when a pulse type signal is received.
However, if the signals INA, INB, INC and IND are DC signals, i.e., if signals fixed to a high or low level are input to the phase detectors 304a and 304b, the outputs of the phase detectors 304a and 304b are fixed according to the previous states of the signals INA, INB, INC and IND, as shown in Table 1.
If each phase detector 304a and 304b is a D flip flop type, the output value cannot be estimated. This is because the output value of each phase detector 304a and 304b is fixed according to the level of a signal which has been input to the phase detectors 304a and 304b in synchronization with a previous clock signal. Accordingly, this inability to estimate the output value deteriorates the stability of the optical reproducing apparatus.
TABLE 1D flip flopSR latchINAINBUPDOWNUPDOWNLowLow——LowLowLowHigh——HighLowHighLow——LowHighHighHigh——LowLow
There are several possibilities that when a DC signal is input to the phase detectors 304a and 304b, the tracking servo operation of the optical reproducing apparatus may be unstable. This could occur when the system is initialized, when a defect area or a burst cutting area (BCA) of the disc is reproduced, etc.
The BCA is provided in the inner area of the disc for copyright protection, a serial number, etc. In the defect and BCA areas, since little or no tracking signal is generated in this area and thus a DC component is input to the phase detectors 304a and 304b, the tracking servo operation of the optical reproducing apparatus becomes unstable.
Typically, to solve the problem, a conventional method of preventing the instability of a tracking servo operation is used. In the conventional method, an offset is controlled when an optical reproducing apparatus is initialized, or the previous output of a controller is held using a servo hold filer when a defect is found.
However, the conventional methods may have problems in that a time delay occurs while the previous output is held, and no area information is obtained in a BCA. Therefore, the tracking servo operation becomes unstable.